1. Technical Field
The present invention relates to a radiation emitting device, and more particularly to a radiation therapy device comprising a radiation source and an aperture plate arrangement located between the radiation source and an object for defining a field of radiation.
2. Description of the Prior Art
Radiation emitting devices are generally known and used for instance as radiation therapy devices for the treatment of patients. A radiation therapy device generally comprises a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam can be an electron radiation or photon radiation (X-rays) beam. During treatment, this radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation. Such a radiation therapy device is described in greater detail in the publication "A Primer on Theory and Operation of Linear Accelerators in Radiation Therapy", U.S. Department of Health and Human Services, Rockville, MD, December 1981. In order to control the radiation emitted toward an object, an aperture plate arrangement is usually provided in the trajectory of the radiation beam between the radiation source and the object. For instance, a wedge-shaped energy dose distribution can be achieved by introducing a wedge-shaped absorption-filter between the radiation source and the object; however, in this case the filter has to be changed in accordance with each desired dose distribution. These dose distributions are commonly defined by isodose curves, measured in water.
It is also known to use a moveable aperture plate in connection with a constant radiation source as a substitute for a conventional wedge-shaped filter.
U.S. Pat. No. 4,121,109 discloses a radiation therapy device having a aperture plate arrangement in which at least one aperture plate is moveable. Further, from an article "Wedge-Shaped Dose Distribution by Computer-Controlled Collimator Motion" in Medical Physics (5), September/October 1978, pages 426 to 429, it is known to use a defined plate motion to obtain a wedge-shaped isodose curve during irradiation. Such a wedge shaped isodose curve is frequently desired in radiation therapy in order to adjust to the anatomical conditions of the treatment subject. The wedge-shaped isodose curve results from the fact that different areas of the radiation field are exposed to irradiation for varying lengths of time. The requisite motion of the plate is caused by an iterative process.
U.S. patent application Ser. No. 07/506,975 entitled "Radiation Therapy Device with Moveable Aperture Plates" by Ernst-Ludwig Schmidt and assigned to the same assignee to the present invention describes a radiation therapy device having an aperture plate arrangement in which at least one aperture plate is moveable and in which in the radiation path a non-moveable filter body is introduced, which has a decreasing absorptivity in the opening direction of the plate. By utilizing this non-moveable filter body, greater flexibility can be achieved in the isodose curves that are to be employed. By this means, it is possible to obtain an isodose curve which, for example, increases in the area to be investigated and then decreases again.
Unlike arrangements which have only an exchangeable absorption-filter body and no moveable plate arrangement, a wide variation in the isodose curves can be obtained with plate arrangements using moveable plates. However, such moving plates are rather heavy and therefore sophisticated motor control systems and motors are necessary for moving the plates according to given accurate speed profiles. Furthermore, since radiation is absorbed in the filter body, the efficiency of radiation use is reduced.